Polymerization catalyst and process



POLYMERIZATION CATALYST AND PROCESS William Frank Gresham and NikolausGeorge Merckling, Wilmington, DeL, assignors to I. du Pont de Nemoursand Company, Wilmington, DeL, a corporation of Delaware No Drawing.Application August 30, 1954 Serial No. 453,145

3 Claims. (Cl. 260-94.9)

This invention relates to novel catalyst systems and more particularlyto catalyst systems useful in the polymerization of ethylene to solidpolymers employing tungsten coordination complexes and to the process ofpolymerizing ethylene using novel tungsten catalyst systems.

Heretofore, it has been Widely known that ethylene can be converted tosolid polymers under very high pressures in the presence of catalystswhich are capable of yielding free radicals under polymerizationconditions.

It has also been known heretofore (U. S. Patents 2,212,155, 2,475,520,2,467,234) that certain metal alkyls and Grignard reagents are capableof initiating the conversion of ethylene to solid polymers through afree radical mechanism at high pressures. Ethylene has also beenconverted to solid polymers in the presence of hydrogenation catalystspromoted with alkali metals or alkali metal hydrides (British Patent536,102).

Generally speaking, Friedel-Crafts type catalysts have not beeneffective for converting ethylene to solid polymers but instead haveresulted in the formation of liquid polymers from ethylene; however, ithas recently been reported that solid polymers admixed with oils can beobtained by polymerizing ethylene in the presence of aluminum chlorideand titanium chloride at elevated temperatures and high pressures andadvantageously in the presence of HCl-binding metals like aluminumpowder (Fischer, German Patent 874,215, Ausgegeben, April 20, 1933).

Redox systems have frequently been disclosed for polymerization ofolefinic compounds. In the past, redox systems have resulted in theformation of highly-branched low density polymers, except at extremelyhigh pressures, at which high density ethylene polymers have beenobtained heretofore. In many of these systems a heavy metal compound wasemployed in combination with a reducing component (cf. U. S. Patents2,380,473 and 2,383,425). While various theories have been advanced asto the mechanism of polymerization in redox systems, the art ofpolymerizing olefins in the presence of such combinations of catalystcomponents has not heretofore advanced to the state at which predictionscould be made as to which pairs of oxidizable and reducible componentsmight give good results in the conversion of ethylene to solid polymersexcept, of course, by further experimentation.

It has been discovered, in accordance with the present invention, thatextraordinary and highly useful effects are produced by reducing a.compound of tungsten, in the manner hereinafter described, and bringingthe resulting mixture into contact with a compound containing ethylenicunsaturation. In specific embodiments it has been found that tungstencompounds containing tungsten combined with radicals (suitable examplesbeing alkoxy radicals, alkyl radicals and radicals which form acids whencombined with hydrogen) can be used effectively in the formation of thecatalyst. The reduction step is generally performed by admixing atungsten compound with a 2,872,439 Patented Feb. 3, 1959 strong reducingagent such as compounds having at least one metal to hydrocarbon bond.It is known that such agents will reduce the valence state of thetungsten to a lower state. It is possible that active complexes oftungsten, which are effective in the polymerization ofethylenically-unsaturated compounds, as disclosed herein, are formed byreaction of the tungsten in a low state of valence with the saidreducing agent. In particular embodiments at sufficient quantity of thereducing agent is added to the tungsten compound to achieve not only thereduction of the tungsten but in addition, to achieve the formation ofthe active complex. This active tungsten complex is believed to containtungsten in certain electron distribution states which give the complexa catalytic activity made use of in this invention. Coordination of thereactive complex with organic components does not necessarily destroyits ability as a catalyst, in fact, it appears that ethylene is capableof coordinating with the active tungsten complex in this manner andquite possibly this phenomenon has a bearing on the mechanism of thepolymerization.

The nature of these coordination complexes is not fully understood, butthey are evidently active catalysts or catalyst components which arecapable of initiating the polymerization of ethylene in an extremelyactive manner to produce solid ethylene polymers. The density of thepolymers obtained through the use of these coordination complexesgenerally exceeds, at least to some extent, the density of polyethyleneprepared by the use of free radical types of catalysts, except thosefree radical polymerization processes which employ such extremely highpressures as to produce abnormally high density polyethylene as comparedwith polyethylene made at moderately high pressure (700-1200atmospheres) by a free radical polymerization process (cf. U. S. Patent2,586,322).

it is believed that the coordination complexes hereinabove described arenovel compounds which have not been employed heretofore in thepolymerization of ethylene. The complexes are difiicult to isolate inthe 'pure state but their presence can be indicated from the chemicalproperties of the mixture produced when a tungsten compound is admixedwith sufiicient quantities of a reducing agent and anethylenically-unsaturated compound.

While the polymerization of ethylene to produce solid polymers in thepresence of catalysts disclosed herein can be carried out under mildconditions, it is preferable, from an economic standpoint, to employmoderately high pressures, suitably from 1 to 200 atmospheres or higher,in order to facilitate the handling of ethylene. Much higher pressures,up to several thousand atmospheres, can be employed, but it is notdesirable to do this in view of the extraordinary activity of thecatalysts at lower pressures. Similarly, extremely low temperatures maybe employed. The preferred temperatures are within the range of about0300 C.

The polymerization of ethylene according to the process of thisinvention, takes place most satisfactorily when the polymerizationmixture is substantially moisture-free and also free of other sources ofhydroxyl groups. Carbon dioxide should also be substantially excluded.As in numerous other ethylene polymerization processes, thepolymerization mixture in the process of this invention is preferablykept free of oxygen, since oxygen reacts with the catalyst. In practicaloperations the oxygen content should preferably be held below 20 partsper million. Certain compounds which are capable of coordinating withthe activated tungsten form coordination complexes which are too stablefor optimum results, and accordingly, the presence of these compoundsshould preferably (although not necessarily) be avoided. In

this category are ketones and esters. Hydrocarbon solvents, on the otherhand, can be used quite effectively as reaction media.

The reagents which are effective for forming the active tungstencatalyst are compounds having at least one metal to hydrocarbon bond. Inthis class may be mentioncd Grignard reagents, metal alkyls or aryls orsimilar organometallic compounds. Compounds having two or more metalatoms directly attached to hydrocarbon are included, e. g.PrMg(alkylene)MgBr. An especially preferred class of reducing agentscomprises hydrocarbonsoluble compounds, such as Sn(alkyl) or compoundsof the formula LiAl(alkyl) where the alkyl groups have about 4 to 18carbon atoms.

The novel catalysts described herein are preferably used in dissolved orcolloidally dispersed form. ac tive tungsten complex may be prepared ina separate and prior step. In the latter case, it is advisable to keepthe active tungsten complex at around C. and free from water and aircontamination so as to avoid the decomposition of the complex.

The invention is illustrated further by means of the following examples:

Example 1.Tungsten hexachloride (.005 mole) was dispersed in 100 cc. ofcyclohexane, and .02 mole of tetrabutyl tin was added. The resultingdeeply-colored solution was introduced into a 325 ml. shaker tube undera blanket of nitrogen, evacuated, flushed with nitrogen andre-evacuated. The shaker tube was then heated to 150 C. and pressured to1000 p. s. i. with ethylene and agitated for 30 minutes. Thereupon, thetemperature was increased to 200 C. and agitation continued for another30 minutes. The resulting polymer was filtered from the reaction mixtureand washed with a methanol and hydrochloric acid mixture, a methanol andaqueous sodium hydroxide mixture, and finally with acetone. The drypolymer produced weighed 21 grams and exhibited a density of 0.95 andcould be compression molded into a tough film.

Example 2.--Tungsten hexachloride (0.005 mole) was dissolved in 100 ml.benzene under a blanket of nitrogen and a solution of phenylmagnesiumbromide (0.04 mole) in diethyl other mixed with 50 ml. of benzene wasadded under stirring. During the addition the temperature of thereaction mixture Was kept at 6 C. The resulting mixture was placed intoa 325 ml. shaker tube. The shaker tube was evacuated, flushed withnitrogen, and re-evacuated, heated to 160 C. and pressured with 2500 p.s. i. ethylene and agitated for 2 hours. The resulting polymer wasfiltered and washed by a procedure described in Example 1. The resultingdry polymer gave a. tough film on compression molding at 190 C. and hada density of 0.95.

It is to be observed that the foregoing examples are illustrative onlyand that numerous embodiments of the invention will occur to those whoare skilled in the art.

The products obtained by polymerizing ethylene with catalystshereinabove disclosed are solid polymers exclusively and are notcontaminated with Friedel-Crafts type of oily polymers.

The activity of the catalyst depends in part upon the nature of thegroups which are attached to the tungsten atom. It is quite possiblethat this elTect depends upon the varying degrees of shielding aroundthe cation, i. e. the shielding power of the group attached to thetungsten influences the activity of the catalyst by influencing thetendency of the tungsten to coordinate. Electrical effects may also playa part in this.

The quantity of catalyst can be varied over a rather asraase Wide range.It is desirable to employ a quantity of catalyst which is at least largeenough to produce a reasonably rapid rate for a reasonably long periodof time. Suitably, the preferred quantity is within the range of .001%to 10% based on the weight of tungsten per unit weight monomer.

The polymers which are made under the conditions hereinabove describedfrequently have such high molecular weights that removal of catalyst bydissolving and filtering is extremely diificult. An effective procedurefor obtaining the polymer in a'clean form is to wash with methanol,acetone-hydrochloric acid mixtures in a Waring Blendor several timesfollowed by washing with acetone and thereafter, if necessary, followedby several acetone-aqueous sodium hydroxide washes and finally byacetone-water wash. The products thus obtained are generally white.While this procedure is highly satisfactory for preparing cleanpolymers, it is to be understood that simpler procedures such as washingwith the reaction medium or treatment with water at elevatedtemperatures will be entirely suitable for various practicalapplications. For some applications it may not be essential to removetraces of catalyst.

The activity of the novel catalyst described hereinabove is of such anature that the catalyst is well suited for polymerization ofethylenically-unsaturated compounds other than ethylene, suitableexamples of other polymerizable olefinic compounds being propylene,butadiene, styrene, cycloalkenes, etc.

The structure of the polyethylene made in accordance with the process ofthis invention evidently is characterized by being a straight chainhydrocarbon with vinyl groups at one or both ends of at least some ofthe molecules. The infrared measurements indicatevery little methylsubstitution and a very small number of vinylidine groups, with littleor no transunsaturation or carbonyl groups.

The ethylene polymers obtained in accordance with the process of thisinvention are highly valuable in numerous applications, especially inthe form of films, molded articles, extruded insulation on wire, etc.

We claim:

1. The process of polymerizing ethylene which comprises contactingethylene at a temperature of 0 to 300 C. in the presence of an inertliquid hydrocarbon with the reaction product formed on admixing atungsten chloride with a quantity of tin tetraalkyl suificient to reducesaid tungsten, said alkyl groups having from 4 to 18 carbon atoms, saidreaction product being employed in a quantity of 0.001 to 10% by weightof the said ethylene, and recovering a solid polymer of ethylene.

2. The process as set forth in claim 1 wherein the tin tetraalkyl is tintetrabutyl.

3. The process as set forth in claim 1 wherein the molar ratio of thetin tetraalkyl to the tungsten chloride is 4.

References Cited in the file of this patent UNITED STATES PATENTSTheoretical Chemistry, volume 11, page 847.

1. THE PROCESS OF POLYMERIZING ETHLENE WHICH COMPRISES CONTACTINGETHYLENE AT A TEMPERATURE OF 0* TO 300*C. IN THE PRESENCE OF AN INERTLIQUID HYDROCARBON WITH THE REACTION PRODUCT FORMED ON ADMIXING ATUNGSTEN CHLORIDE WITH A QUANTITY OF TIN TETRAALKYL SUFFICIENT TO REDUCESAID TUNGSTEN, SAID ALKYL GROUPS HAVING FROM 4 TO 18 CARBON ATOMS, SAIDREACTION PRODUCT BEING EMPLOYED IN A QUANTITY OF 0.001 TO 10% BY WEIGHTOF THE SAID ETHYLENE, AND RECOVERING A SOLID POLYMER OF ETHYLENE.